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EP0581656B1 - Verfahren und Vorrichtung zum Bearbeiten von Signalen die von einem Abbildungssystem erfasst werden - Google Patents

Verfahren und Vorrichtung zum Bearbeiten von Signalen die von einem Abbildungssystem erfasst werden Download PDF

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Publication number
EP0581656B1
EP0581656B1 EP93401918A EP93401918A EP0581656B1 EP 0581656 B1 EP0581656 B1 EP 0581656B1 EP 93401918 A EP93401918 A EP 93401918A EP 93401918 A EP93401918 A EP 93401918A EP 0581656 B1 EP0581656 B1 EP 0581656B1
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EP
European Patent Office
Prior art keywords
frequency
band
module
oblique
radar
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
EP93401918A
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English (en)
French (fr)
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EP0581656A1 (de
Inventor
Eric Normant
Jean-Michel Hermer
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Thales SA
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Thomson CSF SA
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G01S13/904—SAR modes
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G01S13/9004—SAR image acquisition techniques
    • G01S13/9019—Auto-focussing of the SAR signals

Definitions

  • the present invention relates to a method and a device for processing the signals acquired by a mapping system.
  • mapping radars are either airborne or on board satellites, and their mission is to acquire signals from the area we are looking to study by sending pulses to this area at a given recurrence frequency F R. These radars are generally aimed sideways. The Doppler frequency of the acquired signals is then used to determine the distance and the position of the points in the area.
  • the Doppler band Bd occupied by the echo signals is equal to the frequency of recurrence F R of the radar.
  • the resolution in azimuth is proportional to the quantity v ⁇ , v being the speed of displacement of the radar and ⁇ , the temporal resolution, ⁇ being itself inversely proportional to the Doppler band B d occupied by the echo signals.
  • the Doppler band B d which corresponds to the Doppler frequency offset interval during the illumination time of the zone, is much larger than the Doppler band [- F R / 2; + F R / 2] of the radar, and, consequently, the resolution in azimuth is better.
  • the major drawback of spotlight mode is that the length of the imaged area is reduced to the 3 dB width of the lobe of the radar antenna.
  • FIG. 1 The principle of the extended spotlight mode is illustrated schematically in FIG. 1 on which the axis XX ′ represents the trajectory of the radar at substantially constant speed, and the straight line passing through the three points E 1 , E 2 , E 3 , an azimuthal portion of the area explored, parallel to the axis XX '. Acquisition of echo signals in the extended spotlight mode takes place as soon as the radar beam intercepts a point in the zoned. As illustrated in FIG.
  • the beam 1d at time td has just completed the acquisition of the echoes coming from the point E 1 , and just begins the acquisition of the echoes coming from the point E 2 .
  • the radar beam 1 c illuminates symmetrically on either side of point E 2 .
  • This instant t c corresponds to the minimum distance from the radar at point E 2 and to a zero Doppler frequency of the echo signal coming from E 2 .
  • the beam 1f of the radar corresponds to the end of the acquisition of the echoes coming from E 2 , and to the beginning of the acquisition of those coming from E 3 .
  • the length of the imaged area depends on the ability of the antenna to deflect in azimuth. It is also necessary that the rotation of the beam in azimuth provides the desired coverage while respecting the lighting time imposed by the azimuth resolution.
  • the Doppler band B d is larger than the Doppler band [- F R / 2; + F R / 2] from the radar.
  • FIG. 1 illustrating the principle of acquisition in extended spotlight mode, has already been described in the introduction. It will be assumed subsequently that the radar started to transmit at a time t 'prior to t d and corresponding to the start of the acquisition for point E 1 , and continues to transmit until a time t '' corresponding to the end of the acquisition for point E 3 .
  • the acquisition mode which is triggered as soon as a point E i is intercepted by the beam of the radar, and the substantially constant speed of rotation of the beam as and when as the radar moves, it is easy to demonstrate that the acquired signals should each have a quasi-linear Doppler frequency law as a function of time and of the same slope.
  • the characteristic property of the extended spotlight mode is that the preceding laws are between two quasi-linear curves 4a, 4b, defining an oblique band of width substantially equal to the frequency of recurrence F R of the radar.
  • the inclination of this oblique band is caused by the fact that the points E 1 , E 2 and E 3 appear at a different angle of depointing as the radar advances. This inclination is also a function of the speed of rotation of the radar beam.
  • the processing method according to the invention proposes a series of steps which makes it possible to reconstruct the three theoretical lines as we will describe below:
  • the first step of the method according to the invention consists in oversampling the signals acquired at a frequency F proportional to the frequency F R of recurrence.
  • the proportionality coefficient is chosen so as to reconstruct lines 1, 2 and 3 in the oblique band.
  • a frequency F equal to 4F R is sufficient for the case treated.
  • FIG. 3 shows the result obtained in the time-frequency plane at the end of this oversampling step.
  • the oblique band (4a, 4b) in the time interval from t 'to t'', contains only three continuous lines 1, 2, 3 which are the three reconstituted laws.
  • oversampling can be done in different ways:
  • Another method consists in making the Fourier transform of the acquired echo signals, making this transform periodic over period F R over a frequency interval [-F / 2; + F / 2], then go back in time by taking the inverse Fourier transform.
  • the problem is that there is still a lot of stray echoes outside this oblique strip.
  • the second step of the method according to the invention consists in making the oblique band horizontal so as to superimpose it on the useful band [- F R / 2; + F R / 2] from the radar.
  • the result of this second step is illustrated in FIG.
  • the next step according to the invention consists in a bandpass filtering of band equal to [-F R / 2; + F R / 2] so as to eliminate all the spurious signals 1, 2, 3 of which only a few have been shown in dotted lines in FIG. 4.
  • the last step of the method according to the invention consists in restoring the initial inclination to the strip between the lines 4a, 4b. This can be done by multiplying the samples after filtering by a signal whose frequency law as a function of time is a straight line passing through the origin of the time-frequency plane Doppler, and whose slope is equal to the initial inclination of the oblique band.
  • Figure 5 we can see that the theoretical laws 1, 2, 3 have been reconstructed in practice and that there is no longer any possible ambiguity. The spectrum of the starting signals has been unfolded and the ghost targets eliminated.
  • FIG. 6 gives the curves 1 ′, 2 ′, 3 ′ of migration of the respective points E 1 , E 2 , E 3 in the radar distance-frequency plane. These three curves 1 ′, 2 ′, 3 ′ are almost superimposed and have the general appearance of what one would obtain with a SAR radar with conventional lateral aiming.
  • the samples from processing according to the method of the invention can therefore be processed by any conventional SAR processor.
  • FIG. 7 gives a nonlimiting example of a device for implementing the method according to the invention:
  • the video radar signal is first of all processed by an oversampling device 11 at the frequency F.
  • the samples from the oversampling device 11 are then multiplied, by means of a first multiplication module 22, by samples from a first ramp generation module 33, which generates a quasi-linearly frequency modulated signal with a slope opposite to the initial inclination of the oblique band.
  • the samples resulting from this multiplication are then filtered by a bandpass filter 44 of band [-F R / 2; + F R / 2] then are multiplied, by means of a second multiplication module 55 by samples from a second ramp generation module 66, which generates a signal modulated almost linearly in frequency with a slope equal to the initial inclination of the oblique strip.
  • a great advantage of the present invention thus lies in the capacity to transform data providing a radar image with higher azimuth resolution than in conventional SAR, over a larger area than in conventional spotlight mode, into samples capable of being processed by a conventional SAR processor designed for lateral aiming.

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  • Engineering & Computer Science (AREA)
  • Remote Sensing (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • General Physics & Mathematics (AREA)
  • Radar Systems Or Details Thereof (AREA)

Claims (6)

  1. Verfahren zur Bearbeitung von Echosignalen, die von einem Kartographiesystem mit einer Wiederholfrequenz FR erfaßt wurden, wobei das System im erweiterten Spotlight-Modus mit seitlichem Sichtfeld arbeitet und jedes erfaßte Echosignal theoretisch in der zwischen der Zeit und der Dopplerfrequenz definierten Ebene ein kontinuierliches und praktisch lineares, sowie zwischen zwei im wesentlichen linearen Kurven enthaltenes Gesetz beachtet, wobei diese Kurven eine gleiche ursprüngliche Neigung in dieser Ebene besitzen und ein schräges Band definieren und wobei dieses Gesetz in der Praxis aufgrund von Faltungen der Dopplerfrequenzen im Nutzfrequenzband des Systems [-FR/2; +FR/2] diskontinuierlich ist, dadurch gekennzeichnet, daß nacheinander folgende Schritte durchgeführt werden:
    - die erfaßten Echosignale werden mit einer Frequenz F proportional zur Wiederholfrequenz FR mit einem Proportionalitätsfaktor übergetastet, der so gewählt ist, daß das Gesetz in dem schrägen Band wieder kontinuierlich wird,
    - das schräge Band wird in der erwähnten Ebene horizontal gemacht, so daß es das Nutzfrequenzband [-FR/2; +FR/2] des Radargeräts überlagert,
    - man filtert ein Band [-FR/2; +FR/2] aus,
    - man gibt dem schrägen Band wieder seine ursprüngliche Neigung.
  2. Bearbeitungsverfahren nach Anspruch 1, dadurch gekennzeichnet, daß der Proportionalitätsfaktor eine ganze Zahl N ist und daß die Übertastung darin besteht, zwischen die erfaßten benachbarten Echosignale N-1 Werte Null einzufügen.
  3. Bearbeitungsverfahren nach Anspruch 1, dadurch gekennzeichnet, daß die Übertastung darin besteht, die Fouriertransformierte der erfaßten Echosignale zu bilden, diese Transformierte periodisch zu machen mit einer Periode FR in einem Frequenzintervall [-F/2; +F/2] und dann die umgekehrte Fouriertransformierte zu bilden.
  4. Bearbeitungsverfahren nach einem beliebigen der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß für die Umwandlung des schrägen Bands in ein waagrechtes Band die aus der Übertastung stammenden Tastproben mit einem Signal multipliziert werden, dessen Gesetz in der zwischen Zeit und Dopplerfrequenz definierten Ebene eine praktisch gerade Linie ist, die durch den Ursprung der Ebene verläuft und eine entgegengesetzte Neigung zur ursprünglichen Neigung des schrägen Bandes besitzt.
  5. Bearbeitungsverfahren nach einem beliebigen der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß zur Wiederherstellung der ursprünglichen schrägen Lage des Bandes die nach der Filterung verbleibenden Tastproben mit einem Signal multipliziert werden, dessen Gesetz in der zwischen der Zeit und der Dopplerfrequenz definierten Ebene eine praktisch lineare Gerade ist, die durch den Ursprung geht und deren Neigung der ursprünglichen Neigung des schrägen Bandes entspricht.
  6. Bearbeitungsvorrichtung zur Durchführung des Verfahrens nach den Ansprüchen 1 bis 5, dadurch gekennzeichnet, daß sie enthält:
    - eine Übertastungsvorrichtung (11) mit der Frequenz F, die am Eingang die vom Radargerät erfaßten Echosignale empfängt,
    - einen ersten Modul (33) zur Erzeugung einer Frequenzrampe mit einer Neigung entgegengesetzt zur ursprünglichen Neigung des schrägen Bandes,
    - einen ersten Multipliziermodul (22), der die aus der Übertastungsvorrichtung (11) kommenden Tastproben mit der Frequenzrampe multipliziert, die aus dem ersten Modul (33) zur Erzeugung einer Rampe stammt,
    - ein Bandpaßfilter (44) für das Frequenzband [-FR/2; +FR/2], das die aus dem ersten Multipliziermodul (22) stammenden Tastproben filtert,
    - einen zweiten Modul (66) zur Erzeugung einer Frequenzrampe mit einer Neigung gleich der ursprünglichen Neigung des schrägen Bands,
    - einen zweiten Multipliziermodul (55), der die aus dem Bandpaßfilter (44) kommenden Tastproben mit der Frequenzrampe multipliziert, die im zweiten Modul (66) zur Erzeugung einer Rampe gebildet wurde.
EP93401918A 1992-07-31 1993-07-23 Verfahren und Vorrichtung zum Bearbeiten von Signalen die von einem Abbildungssystem erfasst werden Expired - Lifetime EP0581656B1 (de)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR9209533 1992-07-31
FR929209533A FR2694412B1 (fr) 1992-07-31 1992-07-31 Procédé et dispositif de traitement des signaux acquis par un système de cartographie.

Publications (2)

Publication Number Publication Date
EP0581656A1 EP0581656A1 (de) 1994-02-02
EP0581656B1 true EP0581656B1 (de) 1997-03-12

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EP93401918A Expired - Lifetime EP0581656B1 (de) 1992-07-31 1993-07-23 Verfahren und Vorrichtung zum Bearbeiten von Signalen die von einem Abbildungssystem erfasst werden

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EP (1) EP0581656B1 (de)
DE (1) DE69308677T2 (de)
FR (1) FR2694412B1 (de)

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FR2694412B1 (fr) 1994-09-16
DE69308677T2 (de) 1997-06-19
DE69308677D1 (de) 1997-04-17
FR2694412A1 (fr) 1994-02-04
EP0581656A1 (de) 1994-02-02

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